1. Field of the Invention
The present invention relates to a plasma film forming apparatus for activating a raw material for a film, thereby depositing the material to form a film on a surface of a substrate.
2. Description of the Related Art
Films such as protective films and functional films have been widely used on a surface of a substrate composed of a plastic, metal, or ceramic. One known method for forming such films (i.e. film formation) is a plasma film forming method using a plasma.
The plasma film forming method is generally carried out in a plasma film forming apparatus having a chamber equipped with a high-vacuum pump or the like. Thus, a substrate is placed in the chamber, the chamber is evacuated by the high-vacuum pump, a plasma is generated in the extremely low pressure atmosphere, and a raw material for a film is activated by the plasma. As a result, a substance derived from the film raw material is deposited on the substrate, whereby a film is formed on a surface of the substrate.
However, in this case, the high-vacuum pump per se is expensive. In addition, the chamber has to be pressure resistant, so that also the chamber becomes expensive. Therefore, the conventional plasma film forming method is clearly disadvantageous in high equipment cost. Furthermore, the film formation subject is limited to a substrate that can be contained in a chamber.
In view of this problem, a plasma apparatus capable of the plasma film formation under an atmospheric pressure has been proposed in Japanese Laid-Open Patent Publication No. 06-002149. This apparatus is advantageous in low equipment cost because a high-vacuum pump is not needed and the chamber does not have to be pressure resistant. Furthermore, there are no limitations on the shape and size of a substrate for the apparatus.
However, the apparatus is disadvantageous in that an impurity such as air is mixed in a generated plasma discharge gas, whereby the resultant film contains a large amount of the impurity. Therefore, the film has low quality and hardly exhibits the desired function.
This problem may be solved by forming a continuous flow of a plasma discharge gas and a raw material gas toward a film formation region on a surface of a substrate as proposed in Japanese Laid-Open Patent Publication No. 2003-173899. In this case, the flow of the plasma discharge gas and the raw material gas is blocked from an impurity such as air. Thus, the plasma discharge gas and the raw material gas are prevented from mixing with the impurity by a so-called block function. The resultant film can have high quality and exhibit satisfactorily desired function.
In the conventional technology described in Japanese Laid-Open Patent Publication No. 2003-173899, the plasma discharge gas and the raw material gas are each supplied from a separate supply source to the film formation region of the substrate in a direction, and the plasma discharge gas and the unreacted raw material gas transported from the film formation region is discharged in a different direction, so that the continuous flow is formed. It is clear from the above that the unreacted raw material gas is discharged to the outside together with the plasma discharge gas in the conventional technology.
Thus, in the technology, the raw material gas is used in an amount greatly exceeding the minimum amount required to form the film, resulting in high material cost.